A chronic model for experimental investigation of epidural anesthesia in the rabbit

A rabbit chronic model has been developed for investigation of epidural anesthesia without surgery.

A catheter was implanted via a nontraumatic route in the lumbar epidural space between the apex coccis sacri and the first vertebra coccygea after a 1-cm incision was made in the skin at the root of the tail. The ligamentum was punctured to allow the catheter to slide gently 10 cm into the epidural space and avoid subperiosteal soft tissue dissection. The motor block effects of epidural lidocaine and bupivacaine were studied 1 week after implantation. Three injections of lidocaine 1% (group I) or 2% (group II) or bupivacaine 0.25% (group III) or 0.5% (group IV) were given at 48-hour intervals to four rabbits in each group. Increasing concentrations (0.5%, 1.0%, and 2.0%) of lidocaine and bupivacaine (0.125, 0.25, and 0.5%) were injected into eight rabbits in each of groups V and VI. The spinal cord and meninges were examined histopathologically in eight other chronically implanted rabbits that received no drugs.

Catheters were kept in place during 2 months in 40 rabbits without neurologic disturbance, discomfort, infection or weight loss. Complete motor block was observed during a mean 27, 43, and 51 minutes with increasing doses of lidocaine. Administration of bupivacaine 0.125% led to incomplete motor deficit in three rabbits, whereas all other rabbits exhibited complete motor block, lasting a mean of 26, 53, and 93 minutes for increasing doses. Linear relationships were found between the concentration of each drug and the duration of motor block (r = .902, P < .001 for lidocaine; and r = .857, P < .001 for bupivacaine). Repeated injections of local anesthetics produced consistent durations of motor block (Bland and Altman test). No significant histologic changes were observed.

This rabbit model appears to be a suitable tool for evaluating motor block of epidural agents under standardized experimental conditions.